Direct-to-metal ambient rapid-cure coatings with good adhesion and performance characteristics have always been a challenge in the coatings industry. The high throughput rates combined with cure speed is of paramount importance for a metal coater to get the parts done in a fast and timely manner. Various technologies are available of which UV cure coatings is the technology of choice for very high throughput applications. The object of the present invention is to provide a coatings process that can meet the above described challenge.
UV curable coating compositions are known in the art. U.S. Pat. No. 5,684,081 describes a radiation-curable, aqueous dispersion, although the reference is silent as to the wavelength of the radiation to be used. Also known are compositions that are curable using UV radiation having a very low UV-B content and substantially no UV-C content (see, e.g., U.S. Patent Application Publication No. 2003/0059555 and U.S. Pat. No. 6,538,044). The compositions described in the '044 patent are fragranced lacquer coatings that are non-aqueous and are not based on urethane chemistry. The '555 publication describes solvent-based compositions useful as primers. The compositions therein are non-aqueous and require wiping of the coating with an organic solvent following exposure to UV radiation and before sanding of the coated part.
U.S. Pat. No. 6,559,225 describes an aqueous polyurethane dispersion for use in lacquers and coatings. The '225 patent does not describe UV curing, and hints that the dispersions described therein can be combined with radiation-curable binders (column 5, lines 17-20). U.S. Pat. No. 6,579,932 describes an aqueous coating composition which is a mixture of a polyurethane/acrylate hybrid dispersion and a polyurethane resin with oxidative drying groups. The '932 patent does not describe UV curing.
Aqueous radiation curable dispersions are also known (see, e.g., U.S. Pat. Nos. 5,362,773, 6,011,078, 6,479,577, 6,521,702 and 6,541,536).
Non-aqueous, radiation curable compositions are also known. WO 01/74499 describes a primer composition containing one or more compounds containing two or more ethylenically unsaturated polymerizable groups per molecule. Among the compounds described are epoxy acrylates, urethane acrylates and unsaturated polyesters. The compositions described therein also require significant amounts of relatively low molecular weight materials (such as ethylhexyl acrylate and isobornyl methacrylate). The '499 publication indicates that the compositions described therein can be cured with UV radiation using UV lamps having a UV-B:UV-A ratio of 1:1 or less and substantially no UV-C content. As in the '555 publication noted above, the compositions require wiping of the coating with an organic solvent following exposure to UV radiation and before sanding of the coated part. Similar compositions are described in published U.S. Patent Application Publication Nos. 2003/0045598 and 2003/045596, and U.S. Pat. Nos. 4,937,173, 5,013,631, 5,213,875 and 6,509,389.
Radiation curable urethane acrylates are also described in U.S. Pat. Nos. 4,380,604, 6,232,360, 6,753,394 and 6,790,485. Finally, radiation curable epoxy acrylates are described in U.S. Pat. Nos. 5,726,255, 5,756,829, 6,359,082, RE 37,448 and 7,268,172 B2.